1. Field of the Invention
The present invention relates to a set-top box that is installed in the home of a subscriber to a cable television network, and more particularly to a front-end device of a set-top box for two-way communication with a built-in cable modem for performing data communication on a cable television network.
2. Related Art of the Invention
A set-top box for two-way communication which has a built-in cable modem for conducting data communication has been proposed for cable television networks in order to enable services such as video distribution services from the head-end to a subscriber's home, internet services that perform data communication between the head-end and a subscriber's home and, more recently, video-on-demand services that combine video distribution and data communication (for example, see Japanese Patent Laid-Open No. 2004-343239).
FIG. 5 is a view showing the external appearance of a conventional front-end device of a set-top box for two-way communication with a built-in cable modem. FIG. 6 is a view showing the configuration of a cable interface module 41 of the conventional front-end device of a set-top box for two-way communication.
Hereunder, one example of the above conventional front-end device of a set-top box for two-way communication will be described referring to the drawings.
As shown in FIG. 5, a cable interface module 41, a video tuner 42 and a communication tuner 43 are disposed on a main substrate 46.
The cable interface module 41 is provided with a first connector terminal 45a, a second connector terminal 45b, a third connector terminal 45c and a fourth connector terminal 45d. The fourth connector terminal 45d is connected to the video tuner 42 by a first coaxial cable 44a, and the third connector terminal 45c is connected to the communication tuner 43 by a second coaxial cable 44b. The first connector terminal 45a and the second connector terminal 45b are external terminals of the set-top box for two-way communication.
Further, as shown in FIG. 6, the cable interface module 41 comprises a diplexer 51, a first distributor 52, a second distributor 53, a filter 54 and an amplifier 55. The first connector terminal 45a is connected to the diplexer 51, the second connector terminal 45b and the fourth connector terminal 45d are connected to the second distributor 53, and the third connector terminal 45c is connected to the diplexer 51 and the filter 54.
The operation of the conventional front-end device of the set-top box for two-way communication that is configured as described above is described below.
First, in the cable interface module 41, the first connector terminal 45a is connected to the cable television network and the second connector terminal 45b is connected to an audio-visual device in the subscriber's home. Signals of the cable television network that are input to and output from the first connector terminal 45a are subjected to frequency separation by the diplexer 51 such that downstream signals (90 MHz or more) from the head-end to the subscriber's home are separated and sent to the amplifier 55 and upstream signals (55 MHz or less) from the subscriber's home to the head-end are separated and sent to the third connector terminal 45c. 
The amplifier 55 amplifies the downstream signal and inputs the signal to the first distributor 52. The first distributor 52 distributes the power of the downstream signal to output signals to the filter 54 and the second distributor 53. The second distributor 53 distributes the power of the downstream signal again to output signals to the second connector terminal 45b and the fourth connector terminal 45d. Since a downstream signal is output to the second connector terminal 45b, another audio-visual device can be connected thereto for use.
The filter 54 allows only a downstream signal (90 MHz or more) to pass through and outputs the signal to the third connector terminal 45c. Because of the operation of the filter 54, upstream signals of the cable television network only pass between the first connector terminal 45a and the third connector terminal 45c. 
The above is a description of the operation of the cable interface module 41.
Next, the operation of the video tuner 42 and the communication tuner 43 will be described.
The video tuner 42 selects (i.e., tunes to) one channel of the downstream signal and outputs the data to a video signal demodulating part. Meanwhile, the communication tuner 43 selects another single channel of the downstream signal to output the data to a communication signal demodulating part and also mixes the data with data that is input from a communication signal modulating part to supply the resulting data to the third connector terminal 45c of the cable interface module 41.
A front-end device of a set-top box for two-way communication can be configured in this manner. Thus, the front-end device of a set-top box for two-way communication is commonly configured by connecting video tuner 42 and communication tuner 43 that are commercially available with the custom-built cable interface module 41 using coaxial cables.
However, in order to change the front-end device of a set-top box for two-way communication with the conventional configuration into a front-end device of a set-top box without a built-in cable modem, the cable interface module had to be newly designed.
There is no necessity for a set-top box that is dedicated to a video distribution service to include a cable modem for carrying out data communication.
For example, when diverting the front-end device for a set-top box for two-way communication that includes a cable modem, as shown in FIG. 5, for use as the front-end device of a set-top box dedicated to a video distribution service, if only the communication tuner 43 is removed some parts will remain that are not required in a set-top box dedicated to a video distribution service. More specifically, the diplexer 51, first distributor 52, filter 54 and the like of the cable interface module 41 shown in FIG. 6 are parts that are not necessary in a set-top box that is dedicated to a video distribution service. Since costs are increased by these remaining parts, conventionally the cable interface module has been newly designed to create the front-end device of a set-top box dedicated to a video distribution service.
Further, in the conventional front-end device of a set-top box for two-way communication, since the cable interface module 41, video tuner 42 and communication tuner 43 are disposed on the main substrate 46 and connected to each other using the first coaxial cable 44a and second coaxial cable 44b, a drawback existed with respect to an increase in material costs and a rise in the number of assembly man-hours.
Further, the filter 54 is necessary because the second coaxial cable 44b is used for the connection between the cable interface module 41 and the communication tuner 43 in which downstream signals were superimposed with upstream signals.
It is an object of the present invention to solve the above-described drawbacks of the prior art by providing a front-end device of a set-top box for two-way communication that enables a reduction in the number of parts and easily realizes common use for a set-top box without a built-in cable modem.